The present invention relates to thermally-developable imaging materials such as photothermographic materials. More particularly, it relates to photothermographic imaging materials that provide images that are more stable to light and heat over time particularly under light box conditions. The invention also relates to methods of imaging using these materials. This invention is directed to the photothermographic imaging industry.
Heat-developable thermographic and photothermographic imaging materials (that is, heat-developable photographic materials) have been known in the art for many years.
Thermography or thermal imaging is a recording process wherein images are generated by the use of thermal energy. In direct thermography, a visible image is formed by imagewise heating a recording material containing components that changes color or optical density upon heating. Thermographic materials generally comprise a support having coated thereon: (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing system (usually including a developer) for the reducible silver ions, and (c) a hydrophilic or hydrophobic binder.
Thermal recording materials become photothermographic materials upon incorporating a photosensitive catalyst such as silver halide. Upon imagewise exposure to irradiation energy (ultraviolet, visible or IR radiation) the exposed silver halide grains form a latent image. Application of thermal energy causes the latent image of exposed silver halide grains to act as a catalyst for the development of the non-photosensitive source of reducible silver to form a visible image. These photothermographic materials are also known as xe2x80x9cdry silverxe2x80x9d materials.
In such materials, the photosensitive compound is generally a photographic type photosensitive silver halide that is considered to be in catalytic proximity to the non-photosensitive source of reducible silver ions. Catalytic proximity requires intimate physical association of these two components either prior to or during the thermal image development process so that when silver atoms (Ago, also known as silver specks, clusters or nuclei) are generated by irradiation or light exposure of the photosensitive silver halide, those silver atoms are able to catalyze the reduction of the reducible silver ions within a catalytic sphere of influence around the silver atoms [Klosterboer, Neblette""s Eighth Edition: Imaging Processes and Materials, Sturge, Walworth and Shepp (Eds.), Van Nostrand-Reinhold, New York, Chapter 9, pages 279-291, 1989]. It has long been understood that silver atoms act as a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive reducible silver ions in a number of different fashions (see, for example, Research Disclosure, June 1978, Item No. 17029). Other photosensitive catalysts such as titanium dioxide and zinc oxide can be used in place of silver halide.
The photosensitive silver halide may be made xe2x80x9cin situ,xe2x80x9d for example by mixing a halogen-containing source (either organic or inorganic halogen source) with the source of reducible silver ions to achieve partial methasis and thus causing the in-situ formation of silver halide (AgX) grains throughout the reducible silver ion source [see, for example, U.S. Pat. No. 3,457,075 (Morgan et al.), by coprecipitation of the silver halide and the reducible silver ion source [see for example U.S. Pat. No. 3,839,049 (Simons)], or by complete conversion of a portion of the silver ions to the silver halide and adding that portion back to the reducible silver ion source.
The silver halide may also be pre-formed and prepared by an ex situ process whereby the silver halide (AgX) grains are prepared and grown separately in an aqueous or an organic solvent before mixing and/or coating with the source of reducible silver ions. With this technique, one has the possibility of controlling the grain size, grain size distribution, dopant levels, and composition much more precisely, so that one can impart more specific properties to the photothermographic material.
The non-photosensitive source of reducible silver ions is a material that contains silver ions. Typically, the preferred non-photosensitive source of reducible silver ions is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, or mixtures of such salts. Salts of other organic acids or other organic compounds, such as silver imidazolates, silver benzotriazoles, silver benzotetrazoles, silver benzothiazoles and silver acetylides have also been proposed. U.S. Pat. No. 4,260,677 (Winslow et al.) discloses the use of complexes of various inorganic or organic silver salts.
In photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ago) in what is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further developed to produce a visible image by the reduction of silver ions that are in catalytic proximity to the silver halide grains bearing the clusters of silver atoms. This produces a black-and-white image. The non-photosensitive silver source is reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced depending upon the reducing agent in the materials.
In both thermographic and photothermographic materials, the reducing agent for the reducible silver ion of the light-insensitive silver salt, often referred to as a xe2x80x9cdeveloper,xe2x80x9d may be any compound that can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for both thermographic and photothermographic materials. At clevated temperatures the reducible silver ions arc reduced by the reducing agent. In thermographic materials, simply heating above the development temperature is sufficient to cause the reduction reaction. In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. In both thermographic and photothermographic materials, this reaction produces an image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging emulsion.
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing using aqueous processing solutions.
In photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50xc2x0 C. or more is essential for this dry development. In contrast, conventional wet-processed photographic imaging materials require processing in aqueous processing baths to provide a visible image at more moderate temperatures (from 30xc2x0 C. to 50xc2x0 C.).
In photothermographic materials, only a small amount of silver halide is needed to capture light and a different form of silver (for example a silver carboxylate) is used to generate the image using thermal development. Thus, the silver halide serves as a catalyst for the physical development of the non-photosensitive reducible silver ions. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver that, upon chemical development, is itself converted into the silver image, or that upon physical development requires addition of an external silver source. Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the xe2x80x9cchemistryxe2x80x9d for imaging is incorporated within the material itself. For example, they include a developer (that is a reducing agent) while conventional photographic materials do not. Even in so-called instant photography, the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of xe2x80x9cfogxe2x80x9d or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems during the preparation of the photothermographic emulsion as well as during coating, storage, and post-processing handling processes.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the image must be stabilized against further imaging and development. In contrast, the silver halide is removed from photographic materials after development to prevent further imaging (that is during the fixing step).
In photothermographic materials, the binder is capable of wide variation and a number of binders (both hydrophilic and hydrophobic) are useful. In contrast, photographic materials are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic materials require dry thermal processing, they pose different considerations and present distinctly different problems in manufacture and use, compared to conventional, wet-processed silver halide materials. In addition, the effects of additives (for example, stabilizers, antifoggants, speed enhancers, sensitizers and supersensitizers) that are intended to have a direct effect upon the imaging process, can vary depending upon whether they have been incorporated in a photothermographic material or incorporated in a photographic material. Furthermore, certain stabilizers are required in photothermographic materials that have quite distinctive properties, such as those that provide brominating properties (for example, tribromomethyl antifoggants).
The benefits of using such additives in one type of material (for example photographic materials) are not predictive of whether such additives will provide a desired benefit in photothermographic materials. Additives that have one effect in conventional silver halide photography may behave quite differently in photothermographic materials where the underlying chemistry is so much more complex. For example, it is not uncommon for a photographic antifoggant for a silver halide system to cause various types of fog when incorporated into photothermographic materials. Furthermore, some supersensitizers that are effective for photographic materials are inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette""s Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al (Eds.), The Focal Press, London and New York, 1978, pages 74-75, and in Zou, Shayun, Levy and Serpone, J. Imaging Sci. Technol. 1996, 40, pages 94-103.
Problem to be Solved
Medical images are used by radiologists to consider a patient""s condition and to make medical diagnosis. These images are typically viewed on light boxes that are illuminated by fluorescent light and emit heat over time. Some thermally-developable photothermographic materials used in radiology are more sensitive to those light box conditions than others. For example, photothermographic materials that contain what may be defined as xe2x80x9cpolyhaloxe2x80x9d antifoggants, or antifogging compounds that have moieties that include di- or trihalo groups (such as bichloro, trichloro and tribromo groups) tend to be less stable. The images can begin xe2x80x9cbrowningxe2x80x9d prematurely under some light box conditions. Browning may also occur when silver bromides are used as the photosensitive source of silver ions (that is, the photocatalyst) in photothermographic materials. The various causes and mechanisms of image instability in photothermographic materials is not fully understood, so it is unpredictable as to what means can be used to solve these problems.
The use of optical brightening compounds to protect imaging materials from fading, color change or static fogging is well known. Such compounds have also been used as optical brighteners in heat developable materials. For example, GB 1,565,043 (Fuji Photo) describes putting certain optical brighteners in heat-sensitive emulsion layer, subbing layers or in the support itself, and keeping the optical brighteners separate from tribromomethyl antifoggants to avoid interaction between the two types of compounds.
However, it is not predictable as to what compounds may prevent image degradation or browning in heat-sensitive materials because the sources of image instability are not fully understood. What may suppress browning from one source may not accomplish the desired result from another source.
There is a need in the industry for photothermographic materials that provide images that are more stable to heat and light, such as the conditions to which they are viewed on light boxes. Further, there is a need in the industry for a means to reduce browning caused by the presence of polyhalo antifoggants.
The problems described above are reduced using a photothermographic material comprising a support having on one side thereof, one or more layers comprising a binder and in reactive association:
(a) a photocatalyst,
(b) a non-photosensitive source of reducible silver ions, and
(c) a reducing agent composition for the reducible silver ions,
the material further comprising either in the support or in one or more backside layers on the opposite side of the support, or in a layer associated with one of those backside layers, one or more image stabilizing compounds that are present in an amount sufficient to provide an increase in blocking power of at least 0.01 for the combination of the support and the one o r more backside layers and/or associated layers,
wherein blocking power is defined by the following Equation I:                                           blocking            ⁢                          xe2x80x83                        ⁢            power                    =                      -                                                            ∫                  320                  430                                ⁢                                  α                  ⁢                                      xe2x80x83                                    ⁢                                      (                    λ                    )                                    ⁢                                      xe2x80x83                                    ⁢                  j                  ⁢                                      xe2x80x83                                    ⁢                                      (                    λ                    )                                    ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                    λ                                                                                                ∫                  320                  430                                ⁢                                  j                  ⁢                                      xe2x80x83                                    ⁢                                      (                    λ                    )                                    ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                    λ                                                                                      ⁢                  xe2x80x83                ⁢                  
                ⁢                              α            ⁢                          xe2x80x83                        ⁢                          (              λ              )                                =                                    -              log                        ⁢                          xe2x80x83                        ⁢                          (                                                10                                                            -                      A                                        ⁢                                          xe2x80x83                                        ⁢                                          (                      λ                      )                                                                      +                0.01                            )                                                          Equation        ⁢                  xe2x80x83                ⁢        I            
wherein xcex is the spectral wavelength in nanometers (nm),j(xcex) is the irradiance spectrum (W/cm2/nm) of a light box, and A(xcex) is the absorbance spectrum of the combination of the support and one or more backside layers.
In preferred embodiments, the image stabilizing compounds are anthracene compounds, coumarin compounds, benzophenone compounds, benzotriazole compounds, naphthalic acid imide compounds, pyrazoline compounds, or compounds represented by the following Structure I: 
wherein Z is a 2-benzoxazoyl group, a benzothiazolyl group , a triazinyl group, or a benzimidazolyl group, A is a bridging group that forms a continuous chain of conjugated double or triple bonds with the Z group and is most preferably: xe2x80x83"Parenopenst"Cxe2x95x90C"Parenclosest"r or "Parenopenst"Cxe2x89xa1C"Parenclosest"t
R1, R2 and R3 are defined below, n is 0, 1, 2 or 3, p is 1 or 2 and r and t are independently 1 to 10. All of these compounds are described in more detail below.
This invention also includes a method of providing an image comprising:
(A) imagewise exposing the photothermographic material described above to form a latent image, and
(B) simultaneously or sequentially, heating the photothermographic material to provide a visible image.
It has been found that certain image stabilizing compounds on the backside of the support, or in the support itself, or in a layer associated with the backside, of photothermographic materials can improve image stability in those materials, particularly when they are exposed to light box conditions. This improvement is particularly noticeable in photothermographic materials that contain xe2x80x9cpolyhaloxe2x80x9d antifoggants (defined below) in one or more layers. Thus, the browning sometimes seen in images in such materials is reduced or avoided entirely. These results are achieved without adversely affecting the desired sensitometric properties of the materials.
The image stabilizing compounds useful in the invention are present in sufficient amounts to increase the blocking power as defined in Equation I noted above at least 0.01 and preferably at least 0.2 in the combination of the support and all backside layers, or in the combination of the support, all backside layers and in any layers associated with the backside (defined below). This blocking power parameter can be readily determined for a given light box by knowing the irradiance spectrum of a given light box, and the absorbance spectrum of a given support material and all layers on or associated with the backside of that support. The irradiance spectra of two common light boxes (2B and 3C Picker light boxes) are shown in FIG. 7.
It is also desirable that the image stabilizing compounds be used in such a manner that xe2x80x9cyellownessxe2x80x9d is not unsuitably increased in the photothermographic materials before imaging compared to the color of those materials not having the image stabilizing compounds. xe2x80x9cYellownessxe2x80x9d and other color hues can be measured using the CIE lab scale using the a* and b* values (Commission Intemationale de l""Eclairage). The a* value is a measure of redness (positive a* value), and the b* value is a measure of yellowness (positive b*). In the present invention, the type and amount of image stabilizing compound(s) used in the photothermographic materials must be such that the change in b* (xcex94b*) due to their presence is no greater than +10 b* units, and preferably no greater than +4 b* units.